1. Field of the Invention
The present invention generally relates to a high-frequency relay, and more particularly, to a high-frequency relay switching a high-frequency signal.
2. Description of the Related Art
FIG. 1A and FIG. 1B illustrate an example of a high-frequency relay.
A high-frequency relay 1a comprises an electromagnet unit 2 and a contact unit 3.
The electromagnet unit 2 comprises an iron core 2a, a coil 2b wound around the iron core 2a, and a coil terminal 2c connected to the coil 2b. The coil terminal 2c hangs down so as to be connected to a substrate not shown in the figure.
The contact unit 3 comprises a moving contact unit 4, a fixed make contact unit 5 and a fixed break contact unit 6. The fixed make contact unit 5 and the fixed break contact unit 6 are provided at positions opposite both ends of the moving contact unit 4.
The moving contact unit 4 includes an armature 4b provided opposite the electromagnet unit 2. A permanent magnet 4a is mounted to the armature 4b. A moving spring 4c is arranged unitarily with the armature 4b. A moving make contact 4d and a moving break contact 4e are provided at both ends of the moving contact unit 4. The moving contact unit 4 also includes a common terminal 4f hanging down so as to be connected to a substrate not shown in the figure.
The fixed make contact unit 5 comprises a fixed make contact 5a arranged opposite the moving make contact 4d, and a make terminal 5b hanging down so as to be connected to a substrate not shown in the figure.
The fixed break contact unit 6 comprises a fixed break contact 6a arranged opposite the moving break contact 4e, and a break terminal 6b hanging down so as to be connected to a substrate not shown in the figure.
Besides, the relay has a structure in which a contact drive card is provided by bridging and holding the armature 4b and the moving spring 4c. 
With the relay 1a in use, the moving spring 4c moves according to energization of the coil 2b so as to switch between a make state in which the moving make contact 4d contacts the fixed make contact 5a and a break state in which the moving break contact 4e contacts the fixed break contact 6a. This relay may be referred to as a one-point break one-point make structure type.
With such a high-frequency relay as above, that switches a high-frequency signal, an isolation characteristic is the most important of various characteristics required for the signal switching capability.
The isolation characteristic represents a leakage of a signal between a moving contact and a fixed contact (herein below simply referred to as contacts) in a state where the contacts are broken. As a frequency of the signal becomes higher, the leakage increases. The isolation is defined by the following expression and, as a value obtained by this expression becomes larger, the isolation exhibits a better characteristic:                Isolation=−10 log(Pout/Pin) (unit: dB)                    Pout: output power            Pin: input power                        
In order to increase the isolation characteristic, it is necessary to reduce a capacitance between the broken contacts.
Specific methods for reducing the capacitance between the broken contacts include enlarging a clearance between the contacts, and decreasing opposing areas between the contacts.
However, in the example of the high-frequency relay 1a of the one-point break one-point make structure shown in FIG. 1A and FIG. 1B, there is provided a magnetic circuit component which is a floating conductor, such as the iron core 2a or the permanent magnet 4a, in the vicinity of which the contacts 4d and 5a are arranged. Therefore, a leakage of a signal is large between the magnetic circuit component and each of the contacts 4d and 5a, between the magnetic circuit component and each of the terminals 4f and 5b connected to the contacts 4d and 5a, respectively, and between the terminals 4f and 5b. Therefore, the above-mentioned methods, such as enlarging a clearance between the contacts 4d and 5a, are not necessarily effective. In addition, to extremely enlarge the clearance between the contacts 4d and 5a makes an obstacle hindering miniaturization needs for the relay.
There is a method for solving the above-described problems of the relay 1a shown in FIG. 1A and FIG. 1B. In a relay 1b of a two-point break two-point make structure shown in FIG. 2, this method reduces a stray capacitance by grounding a moving make spring 7c via ground terminals 7d upon breaking the contacts. In FIG. 2, reference mark 7a indicates a moving break spring, and reference mark 7b indicates common fixed contacts. In addition, there are relays 1c and 1d shown in FIG. 3 and FIG. 4, respectively, in which grounded shield plates 7f are provided at peripheries of the contact unit. In FIG. 3 and FIG. 4, reference mark 7e indicates a card, and reference mark 7g indicates a resinous base. In these examples, however, the relay 1c shown in FIG. 3 has a structure in which the contact unit and the magnetic circuit are separated; this hinders miniaturization of the relay. Also, as in the relay 1d shown in FIG. 4, the grounded shield plates 7f make a complicated structure; thus, the components cannot be easily manufactured and mounted. It is noted here that, in FIG. 2 to FIG. 4 and other figures illustrating relays, the same elements are basically referenced by the same reference marks, and will not be described in detail.
In addition, for the purpose of further improving the isolation characteristic, there are a relay 1e as shown in FIG. 5A to FIG. 5C and a relay 1f as shown in FIG. 6A to FIG. 6C. In the relay 1e shown in FIG. 5A to FIG. 5C, a metal plating 7h is provided on a terminal-outlet surface 8a located at a bottom part of a resinous cover 8 so that the metal plating 7h electrically connects to the ground terminals 7d projecting from inside, avoiding the contact terminals 4f, 5b and 6b. In the relay if shown in FIG. 6A to FIG. 6C, a large metal plate 7h connected with the ground terminals 7d is so provided as to avoid the contact terminals 4f, 5b and 6b to an extent that the contact terminals 4f, 5b and 6b do not short.
Further, Japanese Laid-Open Patent Application No. 2000-340084 proposes a relay 1g, as shown in FIG. 7A and FIG. 7B, (the body of) which is covered by a metal case (a conductor cover) 9 provided with the ground terminals 7d. 
However, the heretofore-described conventional relays do not necessarily provide a satisfactory isolation characteristic.